1. Field of the Invention
The present invention relates to the compensation of a video image to provide an accurate hard copy reproduction of the tonal luminance differences of the image.
2. Description of the Prior Art
At the present time, in a number of fields, a hard copy reproduction is made from a video image, or the video image is part of the image reproduction chain, and it is desired that the hard copy be an accurate reproduction of the tonal luminance differences of the image. For example, in the medical field a CAT X-ray scan, an ultrasonic scan, an NMR scan (nuclear magnetic resonance) or thermograph image is produced on a video monitor CRT (cathode ray tube) screen for immediate viewing by the physician. Simultaneously the same image is produced on paper by a laser beam or LED (Light Emitting Diode) xerography system in which the laser beam or LEDs form the reproduced image on the photosensitive drum of the xerography system. That xerography system includes the laser beam or LED electronic system to produce the image on the drum, the photosensitive drum to reproduce the image which is on the original CRT screen; and a paper path to pass paper over the drum and transfer the drum image to the paper. The paper copy may be directly viewed by the physician.
Alternatively, a laser-film system is used in which the intensity of a laser beam is controlled and the beam is scanned, raster fashion, over the surface of an undeveloped photographic film sheet, which is subsequently developed using conventional photo film developing methods.
In the printing field it is sometimes desired to make an accurate printed picture from the color image on a video monitor screen. For example, the screen may show a computer-generated image. That image is reproduced on a photosensitive material or directly on a printing plate to print a hard copy that should be similar in tone, luminance and color to the original object or scene.
It has been found that the hard copy image reproduction may vary significantly from the original image on the video monitor screen. Some of the distortions are due to the inaccuracy of the CRT screen and video image reproduction process. That type of distortion has been recognized and compensation methods have been suggested, generally dealing with the problem as it affects an entire group of video monitors. Similarly, the distortions due to the camera have been recognized and treated, generally by improvements in the optics of the camera or overall corrections in video components (brightness, contrast, etc.). However, many of the distortions are not predictable and may vary from day to day and from one device to another.
Set forth below is a discussion of the problems most frequently encountered in producing an accurate picture in the video-to-xerographic or video-to-laser beam processes as they relate to accurate tonal black-white reproduction. The contribution of each problem to the total final distortion of the picture can change in its characteristics periodically and is not predictable. For example, one day the xerographic toner mixture may be incorrect and seriously distort the picture, and the next day it may still be incorrect but have only a minor adverse effect due to partial compensation distortions from other components in the system. In addition, most of the problem-causing effects are non-linear, so that completely compensating for them in a simple direct way is impossible.
The problems with the conventional system are explained in connection with FIG. 1, which is block diagram of a conventional black-white xerographic system. As shown in FIG. 1, the video image is produced by the video source 10, which may be a video camera, a computer graphics output, or a VCR. The video signal is viewed directly on the original monitor CRT screen 11. The same video image reproduced on an internal photosensitive drum 13 in the xerographic system 14. Generally the image on drum 13 is a positive image compared to the image on monitor screen 11. The xerographic system 14 produces the final hard copy 17, preferably on plain (uncoated) paper.
Each step of this conventional process gives rise to unpredictable distortions. The first set of distortions arises in the laser xerographic system. The ratio between luminance values, i.e., the ratio between shades of gray, on the drum 13 may be inaccurate. For example, the drum may be unevenly coated with photoresponsive material or may be aged. In addition, the relationship of the signal voltage applied to the laser beam to produce an output brightness may not be linear. Consequently, the image produced on the drum 13 may not be directly proportional, i.e., accurate, compared to the image on the original monitor screen.
The xerographic system makes a copy using a conventional xerographic process. The density in such copy may not accurately reproduce the differences in the gray scale because the xerographic drum may have a non-linear "characteristic curve" of density against log exposure. The exact shape of the curve varies from one manufacturer to another and even from one drum to another.
When the toner image is transferred from the drum to the paper, distortions may arise from the variability of the process, variations in corona transfer, voltage, and variations in the paper. In those cases in which the copy is duplicated, additional distortions may occur.
In the case where the hard copy is paper and the paper is made into a print using a printer, still other distortions may arise from the dot size of the printing, the spread of the dots and the variable absorption of the ink into the paper due to various types and batches of paper. Also, the perception of the gray scale may differ depending on the type of printing process that is used.
The present invention is particularly directed to accurate reproduction of the luminance differences in value (differences in a gray scale) and absolute luminance from the image on an original monitor black-white video screen. However, in its broader aspects, the invention is also applicable to the accurate reproduction of color images. The invention is directly applicable to color images in the sense that the video screen may be a color CRT screen and the invention will correct for gray scale distortions in reproducing the image on the color screen. In addition, the reproduction of color images has its own set of problems and distortions, aside from black-and-white tonal differences. These color distortions can also be corrected, and their correction will be discussed at the end of the detailed description.
These color and luminance distortions include: (i) that the color of the photo-sensitive dyes, or the ink pigments, or the xerographic color toners do not match the color on the original monitor CRT screen and do not compensate for the color mis-match of the screen phosphors, and (ii) that the color of the color photo-sensitive papers, dyes or printing inks do not match the color of the first hard copy. In addition, the chemistry for color films and color copies is more complex, and more temperature sensitive, than for black-white film and copies, so that variations in the chemistry or temperature cause distortion shifts in the color. A further problem with color, not found in black-white images, is that the perception of color of the object or video screen (by the human eye) differs from the actual color on the film or print.
In U.S. Pat. No. 4,263,001 entitled "Apparatus and Method For Enhancement of Optical Images", in one embodiment, which is not claimed, a video camera is connected to an electronic image modification device which, in turn, is connected to a single frame storage (to prevent feedback) and a monitor CRT.
In U.S. Pat. Nos. 4,492,987 and 4,520,403, both entitled "Processor For Enhancing Video Signals For Photographic Reproduction", the screen of an electronic camera is electronically modified to enhance photographic reproduction. The entire screen is treated as a unit and its brightness or color is changed in accordance with the distortion introduced by a selected photographic film.
In U.S. Pat. No. 4,658,286 entitled "Method and Apparatus For Correcting Distortions In Reproducing Systems", a type of feedback system is described. In one embodiment three photocells look at a corner of the CRT screen having test colors and their outputs are compared to reference colors.